Light-field imaging has attracted wide attention because of its ability to achieve simultaneous three-dimensional (3D) imaging by capturing projections of four-dimensional (4D) light fields and using the 4D light fields to reconstruct 3D volumes as explained in more detail in Levoy, M., Ng, R., Adams, A., Footer, M. & Horowitz, M. Light field microscopy. ACM Trans. Graph. 25, 924 (2006). Traditional light-field imaging systems use arrays of spatially separated microlenses. In these traditional systems, both the lateral and axial information of an object can be recorded simultaneously, but there is an inherent trade-off between the lateral and axial resolutions. For example, conventional light-field microscopy can image large volumes at high speed but suffers from a decreased spatial resolution compared to a conventional microscope.
To improve the spatial resolution, a deconvolution algorithm based on a wave optics model has been developed to reconstruct 3D volumes with high spatial resolution as described in more detail in Broxton, M., Grosenick, L., Yang, S., Cohen, N., Andalman, A., Deisseroth, K. & Levoy, M. Wave optics theory and 3-D deconvolution for the light field microscope. Opt. Express 21, 25418-25439 (2013) and Prevedel, R., Yoon, Y.-G., Hoffmann, M., Pak, N., Wetzstein, G., Kato, S., Schrodel, T., Raskar, R., Zimmer, M., Boyden, E. S., et al. Simultaneous whole-animal 3D imaging of neuronal activity using light-field microscopy. Nat Meth 11, 727-730 (2014). However, the spatial resolutions of the reconstructions using the deconvolution algorithms have been shown to be non-uniform across their depth. To enhance the performance of the light-field microscope, wavefront coding techniques, in which phase masks are added into the optical path, have been proposed in Cohen, N., Yang, S., Andalman, A., Broxton, M., Grosenick, L., Deisseroth, K., Horowitz, M. & Levoy, M. Enhancing the performance of the light field microscope using wavefront coding. Opt. Express 22, 24817-24839 (2014). However, the superior performance of this approach has been demonstrated only in simulation because it is extremely challenging to create the phase mask on each lenslet using conventional optics techniques. A different approach for simultaneous 3D imaging based on multi-focus microscopy and diffractive optical elements was proposed in Abrahamsson, S., Chen, J., Hajj, B., Stallinga, S., Katsov, A. Y., Wisniewski, J., Mizuguchi, G., Soule, P., Mueller, F., Darzacq, C. D., et al. Fast multicolor 3D imaging using aberration-corrected multi-focus microscopy. Nat Meth 10, 60-63 (2013). However, this approach can probe only the image information on a limited number of discrete planes and does not capture the true 4D light fields. Therefore, novel optical elements are highly desired to address the inherent trade-off between spatial resolution and angular resolution in conventional light-field imaging.
Metasurfaces, essentially 2D optical elements, are promising candidates for replacing bulky optical components. A more complete description of metasurfaces are given in Verslegers, L., Catrysse, P. B., Yu, Z. F., White, J. S., Barnard, E. S., Brongersma, M. L. & Fan, S. H. Planar Lenses Based on Nanoscale Slit Arrays in a Metallic Film. Nano Lett. 9, 235-238 (2009); Yu, N. F., Genevet, P., Kats, M. A., Aieta, F., Tetienne, J. P., Capasso, F. & Gaburro, Z. Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction. Science 334, 333-337 (2011); Arbabi, A., Horie, Y., Bagheri, M. & Faraon, A. Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission. Nat Nanotechnol (2015). doi:10.1038/nnano.2015.186; and Lin, D. M., Fan, P. Y., Hasman, E. & Brongersma, M. L. Dielectric gradient metasurface optical elements. Science 345, 298-302 (2014). Gradient metasurfaces include dense arrangements of resonant optical antennas with space-varying properties and offer tremendous freedom in manipulating optical wave-fronts by imparting local, space-variant phase-changes on an incident electromagnetic wave.
Recently, gradient metasurface have been provided using dielectric gradient metasurface optical elements (DGMOEs) as described in Lin, D. M., Fan, P. Y., Hasman, E. & Brongersma, M. L. Dielectric gradient metasurface optical elements. Science 345, 298-302 (2014). DGMOES are capable of achieving high diffraction efficiencies in transmission mode in the visible spectrum. Ultrathin gratings, lenses, and axicons have been demonstrated by patterning a 100-nm-thin Si layer into a dense arrangement of Si nanobeam-antennas. More recently, interleaved metasurface optical elements with multi-functionalities have been designed to achieve a high packing density of distinct optical elements on a surface, without reducing the numerical aperture of each sub-element. In addition to being ultrathin and compact, these multifunctional metasurfaces can provide entirely new functions that are very difficult or impossible to be achieved with conventional optical components.